The 10x Genomics Visium HD (Cytassist) platform provides high-definition spatial transcriptomics, enabling whole-transcriptome analysis with single-cell scale resolution. It uniquely combines histology with spatially resolved gene expression profiling, allowing for detailed localization and quantification of gene expression within the tissue context. Visium HD uses a redesigned slide architecture featuring millions of 2x2 µm barcoded squares, ensuring continuous tissue coverage without gaps, which is a significant improvement over previous versions. This design facilitates high-resolution mapping across entire tissue sections, making it ideal for studies requiring precise spatial information in fields like cancer biology, neuroscience, and developmental biology. The Cytassist instrument simplifies the workflow by enabling the transfer of transcriptomic probes from standard glass slides (including H&E or immunofluorescence-stained FFPE and fresh frozen tissues) to Visium HD slides, broadening sample compatibility. It is a probe-based method targeting the whole mRNA transcriptome.